The ability to generate and manipulate light with electricity by Sir Humphrey Davy 200 years ago sparked a century of development by the likes of Thomas Edison, Joseph Swan, Sandor Just (tungsten filaments), and Irving Langmuir (inert gas instead of vacuum) leading to the establishment 100 years ago of tungsten filament lamps, which as the dominant light source have fundamentally shifted how people live, work, play. However, the efficiency of such light sources is woefully low. For example a 60 W incandescent light is only 2.1% efficient, a quartz halogen only 3.5%, in terms of generating light within the visible spectrum of the human eye. Accordingly today there is a massive worldwide campaign to have incandescent lights replaced wherever possible by compact fluorescent lights (CFL) which have an efficiency of approximately 22% thereby reducing energy consumption significantly.
However, whilst CFLs provide an immediate and visible statement by Governments and other organizations worldwide that they are addressing global climate change, environmental issues etc they are not a panacea. Amongst the disadvantages of fluorescent lights are frequent switching limiting lifetime, health and safety from the mercury content, UV emissions which affect some materials, flicker affecting individuals with autism, epilepsy, lupus, chronic fatigue syndrome, and vertigo, radio interference, operating temperature where efficiency drops with increasing/decreasing temperature from room temperature, non-operation at below freezing, low-luminance requiring long tubes and limiting power output, dimming, and recycling through the phosphor and mercury.
However, a monochromatic solid state light source within the visible wavelength range can achieve in principle an efficiency approaching 100%. Additional to energy consumption such solid state light sources should also reduce consumption of precious metals, reduce recycling as well as address health and safety issues. Beneficially solid state light sources by virtue of their small size, low weight, and low voltage operation can also be employed in a wide range of situations where incandescent or CFL lights cannot. At present niche applications such as holiday decorations in conjunction with indicator lighting in panels, back lighting in LCD displays etc mean that solid state lighting sales today account for only approximately 2% of the current lighting market and will grow to only approximately 3% in 2011.
Despite this solid state lighting is a massive market which according to NextGen Research (“LEDs and Laser Diodes: Solid State Lighting Applications, Technologies, and Market Opportunities”, February 2009, http://www.nextgenresearch.com/research/1001995-LEDs_and_Laser_Diodes) forecasts the overall solid-state lighting (SSL) market will achieve worldwide revenues topping $33 billion by 2013. The illumination segment of the LED market will see compound annual growth rate (CAGR) of nearly 22% in the 2009-2013 timeframe as cities worldwide shift their streetlights to these more energy-efficient and ecologically friendly solutions. However, the majority of this growth will be generated from niche lighting applications including architectural, task lighting, medical and off-grid lighting applications rather than the residential lighting market according to The Strategy Analytics (“LED Device and Material Market Trends”, June 2009, http://www.strategyanalytics.com/default.aspx?mod=ReportAbstractViewer&a0=4788).
As such the majority of the lighting market remains inaccessible despite the considerable research effort and investment have been expended. This arises due to the challenges in realizing suitable LED technologies and devices using conventional quantum well structures, these including the relatively low internal quantum efficiency of these structures, the low light extraction efficiency realized, and relatively high device fabrication costs. Additionally to achieve a “white” LED today the devices will generally employ a phosphor-conversion scheme, which sets the ultimate quantum efficiency of white LEDs to below 65%. In this regard, the high luminescence efficiencies, low fabrication costs, and processibility of semiconductor nanostructures, including quantum dots and nanowires, have made them promising candidates for future lighting devices and the subject of considerable research and development.
With the recent discovery that the band gap of indium nitride (InN) at approximately 0.7-0.8 eV, see for example J. Yu et al in “Unusual Properties of the Fundamental Band Gap of InN,” (Appl. Phys. Lett., Vol. 80, pp. 4741, 2002) and T. Matsuoka in “Optical Bandgap Energy of Wurtzite InN” (Appl. Phys. Lett., Vol. 81, pp. 1246 (2002)), the epitaxial growth and characterization of InN nanowires and whiskers has become the focus of significant research, including for example T. Stoica et al in “MBE Growth Optimization of InN Nanowires” (J. Crystal Growth, Vol. 290, pp. 241, 2006), R. Calarco et al in “GaN and InN Nanowires Grown by MBE: A Comparison” (Appl. Phys. A, Vol. 87, pp 499-503), C-Y Chang et al in “Electrical Transport Properties of Single GaN and InN Nanowires” (J. Elect. Materials, Vol. 35, No. 4, pp. 738 2006), F. Werner et al “Electrical Conductivity of InN Nanowires and the Influence of the Native Indium Oxide Formed at Their Surface” (Nano. Lett. Vol. 9, No. 4, pp 1567-1571, 2009) and J. Grandal et al “Accommodation Mechanism of InN Nanocolumns Grown on Si (111) Substrates by Molecular Beam Epitaxy” (Appl. Phys. Lett., Vol. 91).
When compared to other nitrides with group III elements including aluminum, gallium and boron, InN exhibits the highest electron mobility (4400 cm2V−1s−1 at 300 K), the smallest effective mass, and the highest saturation velocity. These properties make InN an excellent candidate for next generation of nanophotonic and nanoelectronic devices, including chip level nanoscale lasers and high-speed field effect transistors. Additionally, the band gap of InN at approximately 0.7 eV (1750 nm) when compared with GaN at approximately 3.3 eV (370 nm) means that the ternary alloy InGaN can be continuously tuned from ˜0.7 to 3.3 eV, matching almost perfectly to the solar spectrum Therefore, InGaN has also emerged as a promising material for future high-efficiency full solar spectrum solar cells, E. Trybus et al “InN: A Material with Photovoltaic Promise and Challenges” (J. Crystal Growth, Vol. 288, No. 2, pp. 218-224, 2006) as well as for light sources (LEDs).
It should be apparent to one skilled in the art that in order to provide a full solar spectrum solar cell it should be structured so that the material at the front of the solar cell absorbs the shortest wavelengths and progressively longer wavelengths are absorbed by layers within the solar cell towards the lower most surface. As such, the material within a full solar spectrum solar cell should grade from InxGa1-xN where x≈1 to InyGa1-yN where y≈0. As such it is necessary to grow InN nanowires onto the substrate of the solar cell.
Within the prior art InN nanowires have been predominantly grown using the conventional approach of a foreign metal catalyst via the vapor-liquid-solid growth mechanism, see for example J. Li et al in U.S. Pat. No. 6,831,017 entitled “Catalyst Patterning for Nanowire Devices”, L. Romano et al in U.S. Pat. No. 7,344,961 entitled “Methods for Nanowire Growth”, C. Liang et al in “Selective-Area Growth of Indium Nitride Nanowires on Gold-Patterned Si(100) Substrates” (Appl. Phys. Lett., Vol. 81, 22 (2002) and X. Cai et al in “Straight and Helical InGaN Core-shell Nanowires with a High In Core Content” (Nanotechnology, Vol. 17, pp 2330-2333, 2006). They have also been formed spontaneously under nitrogen rich conditions; see for example C-K Chao et al “Catalyst Free Growth of Indium Nitride Nanorods by Chemical Beam Epitaxy” (Appl. Phys. Lett., Vol. 88) and S. Hersee et al in U.S. Pat. No. 7,521,274 entitled “Pulsed Growth of Catalyst-Free Growth of GaN Nanowires and Application in Group III Nitride Semiconductor Bulk Material.”
Whilst the influence of growth parameters on the structural and optical properties of InN nanowires has also been extensively studied. epitaxial InN nanowires grown according to the prior art exhibit tapered morphology, with a large variation in the wire diameter along the wire length, see for example T. Stoica et al in “MBE Growth Optimization of InN Nanowires” (J. Crystal Growth, Vol. 290, pp. 241, 2006) and J. Grandal et al “Accommodation Mechanism of InN Nanocolumns Grown on Si (111) Substrates by Molecular Beam Epitaxy” (Appl. Phys. Lett., Vol. 91), and demonstrated spectral linewidths for these InN nanowires are commonly in the range of 60-100 meV. The extremely large inhomogeneous broadening observed makes it difficult to study the fundamental properties of InN, including the temperature dependence of the band gap and the electron concentration. Additionally, the poorly defined wire geometry leads to uncontrolled electrical and optical properties, severely limiting their device applications.
Further the direct growth of InN on silicon, the most suitable substrate for InN in terms of lattice and thermal mismatches, has been further complicated by the development of an amorphous SiNx layer during the initial stage of growth, see J. Grandal et al. Attempts to adjust these growth techniques, either by the intentional introduction of hydrogen or buffer layers, such as GaN or AlN, have met with limited success.
The prior art whilst demonstrating InN nanowires can be grown has not yet demonstrated them with high quality and constant cross-section nor have they been grown on suitable substrates for low cost semiconductor processing. These developments to date being hindered to a large extent by the relatively low decomposition temperature of InN (approximately 500° C.-550°) as well as the very high migration rate of indium. Additionally, the prior art does not provide an effective means to control the growth and properties of InN nanowires spontaneously formed under nitrogen rich conditions.
Accordingly it would be beneficial to provide a means of growing high quality, uniform InN nanowires directly upon silicon substrates without the requirement for predeposition of a catalyst. It would be of further benefit for the growth rate and properties of the InN nanowires to be controlled through the parameters of the growth process and for the growth to continue despite the growth of the SiNx layer.
As discussed supra graded InGaN nanowire structures would allow full solar spectrum solar cells to be implemented with a single growth process thereby greatly increasing the efficiency of solar cells and reducing their costs. However, for solid state lighting applications the ultimate goal is a high efficiency white LED. However, according to the prior art simple LED structures whilst offering a fairly broad wavelength range operate at relatively low efficiencies and typically three LED devices are required to even cover a substantial portion of the wavelength range to which the human eye responds, the so-called visible wavelength range, which is 380 nm to 750 nm. As such red, green and blue centered LED devices are typically used to create the impression of white. Blue LEDs were the last to be developed based upon InGaN structures. These blue LEDs also form the basis of phosphor based white LEDs. However, increasing the efficiency of LEDs by the introduction of quantum confined structures, such as quantum wells, multi-quantum wells etc also results in a narrowing of the optical emission from the source thereby requiring that number of sources required to “blend” together for the desired white light increases.
It is within this context that semiconductor quantum dots, nanometer sized semiconductor particles which act as a very small “box” for electrons, and potentially the most efficient light sources ever developed have formed the subject of significant research. A specific class of quantum dot is the colloidal quantum dot created by solution phase chemistry. Much of the appeal of the colloidal quantum dot is that it can be readily integrated with other technology platforms at very low cost and that by varying the physical dimensions of the quantum dots they can be made to emit at points across the entire visible spectrum. Accordingly providing colloidal quantum dots with a range of dimensions within the same localized region acts to provide the required multiple sources to “blend” together to provide the illusion of a white light source. Recent work by R. R. Cooney et al entitled “Gain Control in Semiconductor Quantum Dots via State-Resolved Optical Pumping” (Phys. Rev. Lett., Vol. 102, No. 12, 2009) has shown that quantum dots are in fact the most efficient material for generating gain ever measured. The use of quantum dots as white light emitters and in LEDs has also been proposed and demonstrated, see for example S. Sapra at al (Adv. Mater., Vol. 19, pp. 569, 2007), M. J. Bowers et al (J. Am. Chem. Soc., Vol. 127, 2005), S. Coe et al (Nature, Vol. 420, pp. 800 2002), N. Tessler et al (Science, Vol. 295, pp. 1506, 2002), and M. C. Schlamp et al (J. Appl. Phys., Vol. 82, 1997).
However, whilst colloidal quantum dots are themselves efficient an optical emitter employing them can only be efficient if the colloidal quantum dots are optically pumped with an efficient emitter at the appropriate wavelength. As noted supra InGaN nanowire structures can form the basis for very high efficiency emitters which cover the wavelength range from near UV (370 nm) to the near infra-red (1750 nm).
Accordingly it would be beneficial to provide a combination of colloidal quantum dots with the high quality, uniform InGaN nanowires that are grown directly upon silicon substrates without the requirement for predeposition of a catalyst. Such a combination beneficially combines high efficiency InGaN based nanowire LEDs for pumping highly efficient colloidal quantum dot emitters which when formed from multiple dimensions yield emission across the visible spectrum, thereby providing a high efficiency “white” LED.
To date significant progress has been made in demonstrating blue and blue-green LEDs using InGaN to form the third LED within a red-green-blue combination (RGB) approach to forming a white LED. LEDs for the remainder of the RGB combination being manufactured from typically AlGaInP and GaAsP based quaternary semiconductor systems for yellow-orange LEDs and GaAsP and AlGaAs for red LEDs. However, as noted supra InGaN allows bandgap tuning across the visible spectrum and into the UV/infrared. Within the prior art InGaN/GaN LEDs have exhibited very low internal quantum efficiencies in the green, yellow and red wavelength ranges, see for example P. T. Barletta et al (Appl. Phys. Lett., Vol. 90, 2007) and C. Wetzel et al (MRS Internet J. Nitride Semicond. Res., Vol. 10, No. 2, 2005). One of the primary causes for this low efficiency is the strain-induced polarization field in InGaN/GaN quantum wells and the resulting quantum confined Stark effect, which leads to a spatial charge separation.
In this regard, InGaN/GaN quantum dot heterostructures have drawn considerable attention, see for example K. Tachibana et al (IEEE J. Sel. Top. Quantum, No. 6, 475 2000), N. Grandjean et al (Proc. IEEE, Vol. 95, pp. 1853, 2007) and Q. Wang et al (Appl. Phys. Lett., Vol. 93, 2008). This is based upon their providing strong carrier confinement and being identified, although with some debate, as the emission mechanism for the high efficiency InGaN blue and blue-green LEDs and lasers. However, to date three-dimensional InGaN/GaN quantum dot heterostructures obtained by self-organization using Stranski-Krastanow growth or phase segregation induced. In-rich clusters have yielded similar results to bulk planar InGaN/GaN quantum well heterostructures. As such high efficiency long wavelength (>550 nm) emission has been severely limited by the presence of large densities of misfit-dislocations related to the large lattice mismatch (˜11%) between InN and GaN. However, significantly reduced defect densities can be achieved in InGaN nanowire heterostructures, due to the effective lateral strain relaxation, see Y. Chang et al in “Molecular Beam Epitaxial Growth and Characterization of Non-Tapered InN Nanowires on Si(111)” (Nanotechnology, Vol. 20, 2009). This reduced strain distribution also leads to a weaker piezoelectric polarization field.
As noted supra InGaN nanowires offer advantages for LED manufacturing, including high light extraction efficiency and the compatibility with low cost, large area Si substrates, see Y. Chang et al, and can form the basis of either discrete LEDs or LED based white LEDs in combination with colloidal quantum dots. However, to date such nanowire structures have reported extremely low internal quantum efficiency (<10%), due to the lack of effective carrier confinement in the wire axial direction as well as the nonradiative recombination associated with the presence of surface states. However, with the ability to form nearly defect-free InGaN nanowires as discussed supra then it is proposed that InGaN quantum dots directly embedded in InGaN nanowires would provide a route to realizing high efficiency green and red emission sources. Within the prior art no such nanoscale heterostructures have been reported.
According to embodiments of the invention based upon the ability to form defect-free InGaN nanowires directly onto silicon substrates without foreign metal catalysts it would be beneficial to modify the growth process such that nearly defect-free InGaN/GaN dot-in-a-wire heterostructures on silicon could be implemented. Further, by varying the growth parameters to adjust the In composition within the InGaN quantum dots these high efficiency optical emitters (˜45%) may be beneficially tuned to emit within the green, yellow, and amber/red regions of the visible spectrum to compliment the already existing blue and blue-green emission sources. Beneficially the nearly defect-free growth permits the formation of In-rich nanoclusters to form through phase separation within the InGaN quantum dot, such that these high efficiency optical emitters are further implemented by a unique dot-within-a-dot-in-a-wire structure rather than a prior art dot-in-a-wire approach.
Accordingly it would be beneficial to provide a combine multiple quantum dots with the high quality, uniform InGaN nanowires that are grown directly upon silicon substrates without the requirement for predeposition of a catalyst. Such a combination beneficially provides for InGaN/GaN dot-within-a-dot based nanowire optical emitters allowing LEDs with high efficiency to be implemented across the entire visible spectrum.
According to embodiments of the invention therefore high efficiency LEDs for generating high efficiency solid state white light sources can be manufactured based upon forming nanowires using a nearly-defect free InN process upon silicon substrates that does not require a foreign metal catalyst to be introduced or complex precursor gas control processes to achieve growth which would be incompatible with forming either quantum dot in a wire structures or the quantum dot within a quantum dot within a wire structures for increased efficiency.